The earlier effective filing date of U.S. Provisional Application Ser. No. 60/891,816, entitled “Optical Multi-Discriminant LADAR”, and filed Feb. 27, 2007, in the name of the inventor Bruno J. Evans and commonly assigned herewith is hereby claimed. This provisional application is also hereby incorporated by reference for all purposes as if set forth verbatim herein.
1. Field of the Invention
The present invention pertains to remote sensing and, more particularly, to an optical multi-discriminant LADAR imaging system.
2. Description of the Related Art
A need of great importance in military and some civilian operations is the ability to quickly detect and identify objects, frequently referred to as “targets,” in a “field of view.” A common problem in military operations, for example, is to detect and identify targets, such as tanks, vehicles, guns, and similar items, which have been camouflaged or which are operating at night or in foggy weather. It is important in many instances to be able to distinguish reliably between enemy and friendly forces. As the pace of battlefield operations increases, so does the need for quick and accurate identification of potential targets as friend or foe, and as a target or not.
Techniques for identifying targets have existed for many years. For instance, in World War II, the British developed and utilized radio detection and ranging (“RADAR”) systems for identifying the incoming planes of the German Luftwaffe. RADAR uses radio waves to locate objects at great distances even in bad weather or in total darkness. Sound navigation and ranging (“SONAR”) has found similar utility and application in environments where signals propagate through water, as opposed to the atmosphere. While RADAR and SONAR have proven quite effective in many areas, they are inherently limited by a number of factors. For instance, RADAR is limited because of its use of radio frequency signals and the size of the resultant antennas used to transmit and receive such signals. Sonar suffers similar types of limitations. Thus, alternative technologies have been developed and deployed.
One such alternative technology is laser detection and ranging (“LADAR”). Similar to RADAR systems, which transmit radio waves and receive radio waves reflected from objects, LADAR systems transmit laser beams and receive reflections from targets. Systems that both transmit signals and receive reflections, such as RADAR and LADAR, are known as “active systems.” Because of the short wavelengths associated with laser beam transmissions, LADAR data exhibits much greater resolution than RADAR data.
Lasers are also used in “semi-active” laser (“SAL”) systems. With the SAL system, a narrow laser beam is produced and transmitted toward a target. The laser radiation is typically generated and transmitted from a laser designator aircraft manned by a forward operator or by a ground-based operator. The operator directs the laser radiation to a selected target, thereby designating the target. The laser radiation reflected from the target can then be detected by the laser seeker head of a missile, aircraft, or other platform located remotely from both the target and the laser energy transmitter. Because the transmitter is not located on the same platform as the receiver, such systems are not considered “active” systems. Although SAL systems have proven effective, the next generation of receiver platforms are expected to fly to ranges well beyond those of imaging sensors on board the designator platform.
“Passive” systems are also employed. In passive systems, a detector is used to sense energy produced or reflected from the objects in the scene of interest. One example of a passive system is an infrared sensor that detects heat produced by objects. Alternatively, a light sensor, such as an array of photodiodes, may be used to sense the scene light reflected by the objects in the field of view. Passive, multi-spectral detection in narrow spectral bands is recognized to a highly effective approach for target detection in a thermally cluttered or camouflaged environment. Correlated hyper-spectral radiometric measurements in the atmospheric windows in the short wavelength infrared (“SWIR”) and mid-wave infrared (“MWIR”) bands have been shown to be effective in detecting low contrast, partially obscured and camouflaged targets. However, when considered in other applications, passive systems have been found to have a number of limitations. For instance, data provided by passive systems is sometimes difficult to interpret, since a given level of light intensity may indicate an object with low reflectivity, or the same intensity level may indicate a distant object with high reflectivity.
Various combinations of active, semi-active, and passive systems employing different radiation wavelengths have been attempted in the past. Each of these kinds of systems has certain advantages and disadvantages associated with them. For example, active LADAR systems facilitate ranging and imaging from which targets may be identified while SAL systems do not. But, active LADAR systems require lasers, optics, and electronics associated with transmission that SAL systems omit. Passive IR systems also omit these things, can be used for imaging, and omit even the receive optics of SAL systems, but they do not range like an active LADAR systems and cannot be used for homing like a passive semi-active laser-based system. Similarly, because of the wavelengths of the radiation employed, the performance of each technology varies in a given context not only on the mission scenario, but also responsive to environmental conditions. Balancing the constraints and drawbacks with the various technologies with their advantages can be very difficult when trying to combine them.
Thus, these combinations usually have many problems limiting their practicability. For instance, SAL systems do not have the advantages of passive infrared detection while suffering the limitations of active LADAR systems. Combined active LADAR and passive IR systems overcome this problem, but frequently suffer from other problems. For instance, space and weight limitations are usually severe because of the platform requirements on which the systems are deployed. Attempts to combine active LADAR and passive IR systems have problems with these constraints because, e.g., they employ separate optical trains or separate detectors for the LADAR and IR radiation.
In the past, systems have been developed that collect two-color LADAR data or polarized LADAR data. In some cases, the system also collects passive data through separate apertures. The Polarimetric Imaging Laser Radar (“PILAR”) system is an example of a system that has a polarimetric LADAR, a passive MWIR imager, and a visible camera. They all use their own aperture and the data isn't collected simultaneously nor is it registered at the pixel level. In some cases, the system also collects passive data through separate apertures.
The present invention is directed to resolving, or at least reducing, one or all of the problems mentioned above.